Preparation of the tricyclo[5.3.1.1 3,9 ]dodeca‐1,5‐diyl dication: attempts to prepare a novel µ‐H cation

The tricyclo[5.3.1.1 3,9 ]dodeca‐3,7‐diyl dication ( 6 ) was prepared in situ and characterized by 1 H and 13 C NMR spectroscopy. Reaction of this dication with isopentane, NaBH 4 or H 2 , in an attempt to prepare the µ ‐H cation 7 , did result in hydride transfer, but to the ‘outside’ face of the dication, leading to the conventional monocation 8 , or rearrangement products of this. This monocation was independently prepared, and is thermally less stable than the dication, rearranging to the 2‐ethyl‐2‐adamantyl cation ( 19 ) at −60 °C, Δ G ‡  = 16.0 ± 0.6 kcal mol −1 . The 2‐methyltricyclo[4.3.1.1 3,7 ]undec‐2‐yl cation ( 20 ), a likely intermediate in this rearrangement, was also independently prepared and shown to rearrange to 19 , Δ G ‡  = 14.4 ± 0.3 kcal mol −1 , consistent with a stepwise rearrangement process for 8 →[ 20 ]→ 19 . The structures of dication 6 and monocation 8 were investigated by high‐level ab initio calculations, and modeling studies were carried out to look at the factors that allow dications such as 6 to even be prepared. Copyright © 2001 John Wiley & Sons, Ltd.